On RIS-aided NLOS Underwater Wireless Optical Communication Systems

Underwater wireless optical communication (UWOC) is promising for marine high-speed data transmission, which however requires precise alignment between the light sources and photodetectors. The misalignment and obstructed links widely existed in practice due to the water fluctuation and obstacles will significantly affect system efficacy. UWOC systems typically reliance on non-line-of-sight (NLOS) channels for long-distance communication is also affected by these issues. The emerging and cost-effective technology of reconfigurable intelligent surfaces (RIS) can solve these issues by manipulating signal reflection, altering both signal amplitude and direction. However, research on RIS within UWOC remains inadequate, and existing studies often overlook scattering effects. In this study, we investigate the performance of RIS-assisted NLOS UWOC systems in single input single output (SISO) manner by employing a laser diode (LD) as the light source. RIS is employed to facilitate communication over the obstructed NLOS link by adjusting the emission angle of the light source, receiver angle, and RIS position. We propose an enhanced Monte Carlo method to simulate the channel characteristics by obtaining the temporal and spatial distribution of photons on the receiving plane and evaluating the pathloss. Simulation results show the potential of RIS-aided UWOC NLOS communication against traditional scatter-based NLOS communication schemes and demonstrate reduced signal attenuation in RIS-assisted NLOS links, with a higher concentration of photons at the receiver end. The work underscores intelligent reflecting surfaces as an effective means to overcome link blockages in communication. Furthermore, we examine the impact of factors such as link distance and receiver angles on system performance, laying the groundwork for further applications of RIS in UWOC systems.